How to get the duplicate key in a ToDictionary cast?

Question

i have to cast a list to a dictionary in my app but im getting an error saying that "An item with the same key has already been added". But it is a list with more then 5k objects and i need to see wich objects are with the same key. Is there a way to do that? In the message exception i cant get it, so i thought that i can do it using a foreach or something. Any suggestions? Thanks!

EDIT:

   var targetDictionary = targetCollection.ToDictionary(k => k.Key);

This target collection is a generic IEnumerable, and the key i get from a thirdy party database so i do not have access to it. The solution is find the problematic object and tells the supplier about it.

What are the items in your list? How are you currently trying to turn the list into a dictionary? — Daniel Brückner, May 15 '14 at 17:12
Well you could use something like `Distinct` before `ToDictionary` to ensure that you don't have duplicates however your question doesn't provide enough information to solve the problem. — evanmcdonnal, May 15 '14 at 17:13
So you don't need the dictionary, you just want to know the items with duplicate keys, right? — Daniel Brückner, May 15 '14 at 17:28

score 10 · Accepted Answer · answered May 15 '14 at 17:15

You can use LINQ to catch the duplicates. You can then process them as you wish.

create a dictionary that doesn't contain the duplicates

var duplicates = myList.GroupBy(x => x.SomeKey).Where(x => x.Count() > 1);

var dictionaryWithoutDups = myList
    .Except(duplicates.SelectMany(x => x))
    .ToDictionary(x => x.SomeKey);

create a dictionary that contains only the first of each duplicate

var groups = myList.GroupBy(x => x.SomeKey);

var dictionaryWithFirsts = groups.Select(x => x.First()).ToDictionary(x => x.SomeKey);

score 3 · Answer 2 · answered May 15 '14 at 17:23

var badGroups = collection.GroupBy(item => item.Key)
                          .Where(group => group.Count() > 1);

foreach (var badGroup in badGroups)
{
   Console.WriteLine("The key {0} appears {1} times.", badGroup.Key, badGroup.Count());

   forach (var badItem in badGroup)
   {
      Console.WriteLine(badItem);
   }
}

var goodItems = collection.GroupBy(item => item.Key)
                          .Where(group => group.Count() == 1)
                          .SelectMany(group => group);

foreach (var goodItem in goodItems)
{
   Console.WriteLine("The key {0} appears exactly once.", goodItem.Key);
}

var dictionary = goodItems.ToDictionary(item => item.Key);

paparazzo · Answer 3 · 2014-05-15T17:50:20.177

1

If you are just looking for dups

HashSet<string> hs = new HashSet<string>();   
foreach(string s in myList) if(!hs.Add(s)) Debug.WriteLine("dup: " + s);

Or you could change hs to Dictionary if you want to process

Dictionary<string, myclass> dl = new Dictionary<string, myclass>();    
foreach(string s in myList)
{
   if(dl.ContainsKey(s)) 
   {
      Debug.WriteLine("dup: " + s);
   }
   else 
   {
      dl.Add(s, null);
   }
}

I see you accepted a LINQ answer but LINQ is not going to out perform this.

edited May 15 '14 at 17:50

answered May 15 '14 at 17:19

paparazzo

44,497
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@NathanA yes i just looked the up myself – paparazzo May 15 '14 at 17:24
1

You can simplify this to `if (!hs.Add(s)) { Debug.WriteLine("dup: " + s); }` because `Add` will tell you if the item was already present. – Daniel Brückner May 15 '14 at 17:32
@DanielBrückner Agree but Dictionary.Add does not work that way and I wanted code to go both ways. – paparazzo May 15 '14 at 17:47

score 1 · Answer 4 · answered May 15 '14 at 17:45

1

If you want to keep the duplicates you can use .ToLookup instead. It creates a ILookup<TKey, TValue> which is basically a read-only Dictionary<TKey, IEnumerable<TValue>> where the duplicates are stored as the "value" in a collection.

answered May 15 '14 at 17:45

Scott Chamberlain

124,994
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How to get the duplicate key in a ToDictionary cast?

4 Answers4